The present invention relates to a measuring device for the quality control of printed products.
For a long time, densitometry has been used for controlling the quality of printed products and, in particular, of offset printed products. According to typical densitometry measurement techniques, a measurement field is illuminated at an angle of 45xc2x0 degrees to the surface of the field and reflected light is detected by spectrophotometer at an angle of 0xc2x0 degrees. This is commonly known as 45/0 geometry. The converse geometry is known as 0/45 geometry.
In one technique, radiation derived from the measurement field includes the color of the field is obtained and used to determine the absorption of radiation in the measurement field. The spectrophotometer signal can be converted into densitometry values by a photoelectric converter and a downstream evaluation unit.
This technique of measuring color density, which is widely used in offset printing, is practical for glossy printing since there is a relationship between the measured optical density and the thickness of the ink on the printed sheet. Therefore, these densitometry values are amenable for controlling the feeding of ink.
Because of the angles of incidence, the reflected light may include the gloss of the wet ink. This xe2x80x9cglossy effectxe2x80x9d of the wet ink often affects the accuracy of the measurement unless there is some compensation for the effect. In order to suppress the glossy effect in freshly printed inks, the polarization planes of polarization filters are in a crossed arrangement in the illumination and reflection channels. By suppressing the glossy effect in the light reflected from freshly printed inks, the measured values from fresh and dried inks can be equated.
In addition to measuring the densitometry values on a printed product, measuring calorimetric values have been increasingly used for controlling the quality of printed products. Colorimetric values are measured by either a tristimulus based colorimeter or by a spectrophotometer combined with digital weighting. Colorimetric values are used to control inks of both scaled colors and special colors, and also are used to attain expected color loci.
Colorimeters based on the tristimulus method are compact and simple in structure, but are suitable only for colored samples. In a spectrophotometer, however, reflected light from the measurement field is split up and digitally weighted according to a pre-selected function, thereby providing for any desired filter curve. Thus, for a spectrophotometer, both colorimeter and densitometry values can be obtained, depending on the filter curves applied.
The standards on which the color measurement is based do not provide for the use of polarization filters, because of the intended simulation of the human visual sense. Consequently, spectrophotometers that function as colorimeters and densitometries are either not equipped with polarization filters, or are equipped with switchable polarization filters in a complicated way, which increases the constructional outlay or measurement time considerably.
German patent no. DE 88 16 978 U1 discloses a measuring instrument, which can be moved in the X- and Y-direction, such that any desired locations on printed sheets can be measured. The measuring instrument has two measuring heads. One of the heads is for measuring density and the other is for measuring color. A three-color simultaneous measuring head measures the ink density and a tristimulus-based measuring head measures the color. Neither the three-channel densitometry measuring head or the tristimulus-based measuring head provide a purely spectral measured value. Thus, the use of this equipment is restricted.
In German patent no. DE 195 30 185 A1, a device for measuring the color of prints in offset printing includes an illumination channel with a polarization filter for illuminating the measurement location perpendicularly. Receiving channels, with or without a polarization filter, receive the light reflected from the measurement point at an angle of 45xc2x0 degrees. Although this device is compact in design and construction, it has the disadvantage of requiring the illuminating light to be polarized. For colorimetric values to be derived form the reflected light, however, it is generally not possible to use measured values obtained with polarized light in the irradiation. Thus, this measuring device is also limited in its applications.
According to the invention, a measurement device is provided whose applications to quality control techniques for off-set printing is significantly enhanced with respect to the devices previously known. The measuring device includes two illumination channels that operate simultaneously, but have different optical properties. One of the channels includes one or more polarization filters to remove the xe2x80x9cglossy effectxe2x80x9d of wet ink and the other does not. The illumination channel with polarization filters is associated with a receiving or reflection channel that likewise includes polarization filters. These filters are conventional linear polarization filters. The illumination channel without the polarization filters is associated with a receiving or reflection channel that is also without polarization filters.
A location on the measurement field of a print is measured for ink thickness using the reflection spectra of the reflection channels having the polarization filters and measured for calorimetric values using the illumination and receiving channels without polarization filters. These channels operate simultaneously and eliminate the need for switching polarization filters and the associated successive measurements necessitated by this switching.
Several advantages immediately derive from the simultaneous use of two separate channels. First, both spectra and colorimetric values can be obtained for the measurement fields for the non-polarized channel. Second, by measuring at the measurement location with both polarized and non-polarized channels, spectral measured values are obtained that can be used to assess and compensate for the drying properties of individual inks and overprints. Third, any desired filters can be generated in the non-polarized receiving channels. Therefore, not only scale colors but also special colors can be controlled and/or measured in the respective highest absorption range.
The illumination channels are fed from a central lamp unit. The channels comprise optical waveguides and optics that illuminate the measurement locations. In one channel, a polarization filter is arranged in the beam path of the respective illumination optics. The illumination of the measurement locations in both illumination channels is preferably carried out at an angle of 45xc2x0 by conventional annular optics.
Radiation reflected from the measurement location is fed via the optical waveguides to evaluation units, which are provided with opto-electrical converters. Since the illumination of the measurement field takes place at a 45xc2x0 angle, the reflected radiation is perpendicular to the field. Thus the reflected channels are positioned accordingly in order to capture the reflected radiation. Arranged upstream of the optical waveguides are suitable optics for picking up (e.g., focusing) the radiation reflected from the measurement field.
The reflection channel that includes a polarization filter passes the reflected radiation to an evaluation unit such as a spectrophotometer. Because the reflected light has been compensated for the xe2x80x9cglossy effect,xe2x80x9d the evaluation unit can accurately determine ink density using convention evaluation techniques. Also, in the evaluation unit, the filtered radiation from the central lamp unit can be analyzed in order to compensate, in a well-known manner, for the influences of the lamp spectrum.
The radiation received through the reflection channel without a linear polarization filter is likewise fed via an optical waveguide to a spectrophotometer or a standard colorimetric evaluation unit. The colorimetric evaluation unit analyzes the radiation spectrally using conventional techniques (e.g., numerical weighting of the individual spectral regions).
Thus, the measuring head of the invention provides for the simultaneous collection of data from which both color and ink-density analyses are performed.
Additional features and advantages of the invention will be made apparent from the following detailed description of the illustrative embodiment that proceeds with reference to the accompanying FIGURE.